[Lo0n]

generally i think he got the info from test pilot reports, military test facilities, technical reports, that kinda thing. there are some accounts where they divert from what we see, but then those can be put down to subjective interpretations by pilots in their reminiscences. there are some things, like the high speed elevator lock up in p-38's, that has been shown to be a bit out, but things like pilots saying they easily out turned spitfires in a 109 were down to being an ace, who evidently survived the war to write his books, fighting a novice who didn't fly his plane to it's limits. teh tech data would give a lto different results compared to the accounts of survivors and experts who would be able to wring out performance better than a novice. the he162 tested by eric brown (this is from a recfent post at ubi) had a factory limit of 750kph, and brown took it to 800+ quite happily without snappin the wings off or frying the engine. certainly how the sim handles high speed is odd, ditto high altitude, but then thats a limitation of an old sim originally designed as a study in a single type - the il2 series.
i only ask after the ffb as i don't have it, but imagine it should be somethng that is tied into stalls or would give some indication? from what i recall the effect in spits was a gentle thrubbing as stall approached, something felt in the seat, and the controls would go a bit sloppy but it was mainly the structural noise that warned pilots. quite gently if i'm remembering correctly
for more info on say lockheed, focke wulf and hawker aircraft, look for cwos, they hace sections for each with a load of tech data.